Image-stabilized optical instrument, such as telescope and cameras

ABSTRACT

An image-stabilized optical instrument, such as telescopes and cameras, having an objective (1) and first (2) and second (6) optical reflecting elements following in the path of rays after the objective and, wherever applicable, an ocular (9) arranged after said elements, is characterized by the fact that the whole objective, or one or more parts of the latter, depending on the purpose and the optics used, together with the first optical element (2) forms a stabilized unit, which is freely rotatable, in relation to the casing (7) and the second optical element (6) fixed thereto and, wherever applicable, the ocular (9), around two pivot axes which usually intersect. The suspension point (A,B,C) then substantially lies on the imaginary extension (L) of the optical axis of the objective after the first optical element, if the second optical element would not have been in the path of the optical axis.

The present invention refers to an image-stabilized optical instrument,such as telescopes and cameras.

It is generally known that telescopes with a high magnification cannotbe held by hand without difficulty because the observed image is toounsteady to be capable of being tracked accurately. This is due to thefact that the unavoidable turning movements of the telescope around allaxes orthogonal to an original visual line cause corresponding butgreater movements of the image. This problem increases with increasingmagnification. A method of handling this is to isolate the telescope asa whole from such movements (inertia stabilization) for example by servomechanisms, gyro devices or the like. Common features of these solutionsare that they are complicated, increase the weight and require aconsiderable operating power. Therefore, a great many solutions havebeen presented where only a part of the optics is stabilized and wherethe casing of the telescope and at least the ocular remain unstabilized.The optics is dimensioned so that the stabilized element functions as a"reference element", i.e. the telescope image follows the turningmovements of said element. If this element is stabilized, the image alsoappears stabilized.

When studying existing solutions it is seen that image stabilizationaccording to the abovementioned method can take place substantially inthree different ways with regard to the position of the stabilizedelements.

(1) Stabilization takes place before the objective, outside thetelescope. This can take place by variable prisms or by mirrors. Thecomplexity will here be great, since these "extra" optical elements arenot needed in ordinary telescopes. Also, these elements must be madelarge and with great precision.

(2) The objective is stabilized with the pivot point situated in or nearthe focal plane. This solution has up to now been unpractical owing tothe fact that in practice such an instrument will be very large. Imagereversal by mirrors or prisms is also unsuitable here.

(3) The objective is unstabilized and the stabilizing elements arewithin the telescope. Most existing solutions belong to this group. Aconsiderable disadvantage is common to them all, namely that highdemands are made on the off-axis exactitude of the objective, ifaberrations are to be avoided when the telescope is turned in relationto the visual line. Most instruments of this group also requirecomplicated optics.

Moreover, many solutions of stabilized monocular instruments have beenpresented, even if few if any of them can be made simple and at the sametime with good data. Nevertheless there are few solutions of stabilizedbinoculars described, and these are as a rule very complicated. This hasto do with the fact that it is very difficult to find a solution whichwill give an exactly equal degree of stabilization for the two parts ofthe binoculars, to that doubling of the images or blur does not arise.Generally, all monocular solutions can be converted into binocular ones,if the two stabilized optical elements of the left and right parts ofthe binoculars are connected by mechanical bridging means. This is madeaccording to the U.S. Pat. No. 2,829,557 which requires very greatprecision in a purely mechanical respect, and this, together with thegeneral mechanical complexity with many pivot axes, makes this type ofsolution impractical. Consequently, there is a need of a solution wherea single part stabilizes the image in both parts of the binoculars. Suchsolutions are known, but up to now they have been very complicated in apurely optical respect. In the U.S. Pat. No. 3,460,881 such a solutionis shown, where a plurality of mirrors are provided but where the imagereversal finally takes place with lenses. Another similar solution isshown in the U.S. Pat. No. 3,915,550, where besides some notes are setforth quite generally about stabilizing binocular instruments and aboutproblems in connection therewith. Solutions have also been presentedwhere beam splitting takes place after the stabilized element. Thisarrangement certainly allows using both eyes, but the light intensitynecessarily becomes reduced and no stereoscopic vision is obtained.

From the above it is seen that no simple and at the same time wellfunctioning solution has been known up to the present time, neither formonocular nor for binocular instruments.

The purpose of the present invention is, among other things, to realizemonocular and binocular instruments with image stabilization without anyof the disadvantages mentioned above.

This is possible by the invention having the characteristics defined inthe claims.

An advantage of the invention is that the objective is stabilized whichhas a result that the introduction of image stabilization makes nofurther demand on the objective, compared to an unstabilized instrument;accordingly, standard objectives may be used with advantage.

Among other advantages the following are noted: the optics may be variedin a great many different ways. Only the four reflecting surfaces neededfor image reversal are necessary in the simplest construction oftelescopes. The stabilized part has a high moment of inertia in relationto the length of the instrument. The pivot point can be placed bothwithin and outside the beam of rays from the objective, which results inthe greatest freedom in the choice of bearing being obtained. A simpletype of stabilized binoculars is made possible without the disadvantagesfrom which other structures of similar types have suffered up to now.

Other advantages and characteristics of this invention will be seen fromthe following specification. Although the latter emphasizes thestabilization of the telescope image, it is obvious that opticsconstructed according to the invention may be used without any hindrancein all connections where a focused image is to be stabilized around atleast one axis. Another conceivable sphere of utilization is thestabilization of laser light.

For the best possible understanding of the invention some conceptsfrequently occurring later should be explained more closely here.

The angle which the telescope casing is turned at a certain moment inrelation to the stabilized part is here referred to as angulardeflection. When the angular deflection equals zero, the stabilized partis in its central position. By the "optical axis" is henceforth meantthe line which is defined by the optical axis of the objective lenssystem when the angular deflection equals zero. Finally, the differencebetween dimensioning for telescope stabilization and camerastabilization must be elucidated. In the case of image stabilization ofa camera the focused rays from the lens are to be stable in relation tothe film plane, irrespective of the movements of the camera. In the caseof image stabilization of a telescope the image is to be seen stable inrelation to the environment, which requires a small modification of thedegree of stabilization, compared to dimensioning for 100% camerastabilization. How this is achieved for various embodiments of thisinvention will be shown later, but generally optics dimensioned forcamera stabilization may be converted into telescope stabilization bymodifying the degree of stabilization with a factor (1±1/F), where F isthe magnification of the telescope and where the minus sign applies toinstruments in which the image is seen upright.

In order that an image-stabilized telescope may be usable severalrequirements in respect to the mechanics must be satisfied. The centreof gravity of the stabilized part must be situated at the pivot point, aweak spring force must tend to drive the stabilized part into a centralposition, and a viscous damping must be provided so that resonanceoscillations are damped. This method of stabilization, being well knownpreviously, will not be described further in detail here. Yet, it may bepointed out that stabilization can take place also by means of a gyro.This offers very good stabilization qualities but requires morecomplicated mechanics at the same time as the demand for operating poweris a disadvantage.

Since the stabilized part of optics constructed according to thisinvention has a high moment of inertia in relation to the length of theinstrument, the firstmentioned simpler method of stabilization without agyro is very suitable here.

Generally, the description of mechanics in the following specificationis very summary, partly because it is non-essential to the invention howthe mechanics is designed in detail, partly by the circumstance that thedesign of such mechanics in connection with image stabilization iswell-known. Yet, it should be noted that since the stabilized part incertain embodiments of this invention has an irregular shape, particularattention must be devoted to minimizing the inertia product around allaxes orthogonal to the visual line.

The invention will be described more in detail in the following in theform of examples with reference to the drawing in which FIGS. 1 to 3show the principle of the invention applied to a telescope,

FIG. 4 diagrammatically shows the use of a certain type of opticalelements in a telescope,

FIG. 5 shows an element included therein,

FIG. 6 diagrammatically shows the use of a second type of opticalelements and

FIG. 7 another type of optical elements in connection with theinvention, and

FIGS. 8 and 9 diagrammatically show the invention applied to a pair ofbinoculars,

FIG. 10 shows an alternative to the latter, and

FIG. 11 shows another alternative to the application of the invention inthe case of a pair of binoculars.

The general principle of the present invention is explained inconnection with FIGS. 1, 2 and 3 diagrammatically showing the inventionin connection with a telescope. The stabilized part consists of theobjective lens 1 together with the optical element 2 which may consistof any conceivable combination of lenses, prisms, mirrors and the like,but a requirement is that the element contains at least one reflectingsurface. It should be noted that this stabilized part is the mostimportant one for the invention. Its form can be varied in a great manydifferent ways and other optical elements may be added, but this part isfound in some form or other in all variants.

The objective 1 and the optical element 2 are rigidly fixed to eachother by an arm or bridging member 3 to which two suitably dimensionedcounterweights 4 and 5 may advantageously be fixed. A second opticalelement 6 is, like the ocular 9, rigidly fixed to the casing 7 of thetelescope and is accordingly unstabilized. Light rays from an observedobject first passes the protective plane window 8 and then the objective1, thereupon passes the elements 2 and 6 and finally the ocular 9. Thestabilized part is rotatably mounted, substantially without friction, ata pivot point 30 (not shown in FIG. 1), the exact position of which willbe discussed later. In the case of stabilizing a telescope image,freedom of rotation exists about all axes at right angles to the visualline but not around the latter. The function in general is bestunderstood if FIG. 2 is considered which shows the same telescope asFIG. 1 but an angular deflection α has now taken place; thus the casing7 has been turned the angle α unintentionally around the pivot point 30.The position of the stabilized part remains the same as is shown inFIG. 1. From this it follows that the position of the light rays afterreflection in element 2 is independent of the position of theunstabilized optics. If the pivot point is correctly positioned, theunstabilized element 6 directs the light towards the ocular 9 in such away that a stable image is obtained, irrespective of the angulardeflection.

Provided that the light from the observed remote object at first onlypasses fully stabilized optics and then only passes unstabilized optics,the following may be shown: if the pivot point is situated exactly wherethe focus A of the objective (FIG. 1) would have been located if theunstabilized element 6 would not have been there, dimensioning forcamera stabilization is always obtained irrespective of how the opticalelements 2 and 6 are constructed. Accordingly, these may be constructedas porro prisms, plane mirrors, cube corner elements or the like, andthey may also, as long as at least one reflecting surface is alsoincluded, contain lenses. The abovementioned rule, with the conditionmentioned above, is of universal validity.

The telescope shown in FIGS. 1, 2 and 3 does not entirely satisfy theabove requirements; the covering disc 8 is obviously unstabilized, butthe optical effect of this window may be neglected if its surfaces areplane and parallel.

In the case of dimensioning for telescope stabilization the pivot pointmust be displaced a little from the point which affords camerastabilization (point A) along the line defined by the optical axis,where this would have been situated if the unstabilized element 6 issupposed removed. To obtain a maximum of clearness of the presentationthis line will henceforth be called line L and is shown dashed in allfigures. How far and in which direction the pivot point is to bedisplaced from point A long line L depends on the focal length f_(ok) ofthe ocular and how the unstabilized optical element 6 is constructed,provided that this element does not contain any lenses. It may be notedthat if the unstabilized element 6 contains one or more lenses thelength f_(ok) has to be replaced by the length D, where D equals thefocal length of the objective divided by the magnification of theinstrument. If there are no lenses between objective and ocular, thenobviously D=f_(ok). Some special cases are described below where forsimplicity no lenses are included beween objective and ocular.

In FIG. 4 the essential parts of a telescope are shown, where athreefold reflecting prism 10 is included, shown more in detail in FIG.5. This element has the property of reflecting the light non-rotatedabout the optical axis 11 like a plane mirror but translated the lengthE, provided that the reflecting planes 12, 13 and 14 are oriented suchthat each of them contains a line which is parallel to a line in theother planes. For a more detailed description the U.S. Pat. No.3,475,073 is referred to in which other alternative elements withequivalent optical properties are also shown. Furthermore, in thistelescope a plane mirror 15 and a roof prism 16 are included which areoriented so that the three reflecting surfaces thereof are at rightangles to each other. These surfaces essentially form a cube corner, andhenceforth the plane mirror 15 and the roof prism 16 will, accordingly,together be regarded as a cube corner element 17.

Light from the observed object passes the window 18 and the objective 1,is then reflected in the cube corner element 17 stabilized together withthe objective before passing the ocular 9 after threefold reflection inthe unstabilized prism 10. As seen, the telescope shown in FIG. 4 hasexactly the same fundamental construction as the one described inconnection with FIGS. 1 and 2; the threefold reflecting element 10 inFIG. 4 thus corresponds to the element 6 in FIG. 1, and the cube cornerelement 17 in FIG. 4 corresponds to the element 2 in FIG. 1.Consequently, in the case of dimensioning for camera stabilization thepivot point is to be placed where the focus of the objective would havebeen in the absence of the unstabilized prism 10, point A. In the caseof dimensioning for telescope stabilization the optical properties ofthe unstabilized element 10 require that the pivot point is here placedat point C; the pivot point is thus displaced from point A along theline L the length f_(ok) towards the observer. It should be noted thatimage reversal is also obtained if the two elements 10 and 17 changeplaces; thus still another variant is possible which includes theseelements, which can be image stabilized according to the invention andwhich at the same time will give image reversal, naturally the one inwhich the cube corner element 17 is unstabilized and the prism accordingto FIG. 5 is stabilized together with the objective. All essentialproperties characteristic of the telescope shown in FIG. 4 are foundalso in this new variant, except the placing of the pivot point in thecase of dimensioning for telescope stabilization. In this case the pivotpoint has to be displaced the length f_(ok) along the line L from thepoint A but from the observer, so that in this variant it is situated atpoint B. This point B is found, like points A and C, also in FIGS. 1 and6.

If the optical elements described above are used, the light is reflectedsix times between objective and ocular. This may seem to be anunnecessary complexity compared, for example, to a porro system whereonly four reflections occur. However, such a system may also be usedwith advantage in connection with image stabilization according to thisinvention, which is described in connection with FIG. 6. Here theessential parts of a telescope are shown, containing four mirrorsarranged in a well-known way for image reversal. The two mirrors 21 and22 at right angles to each other are rigidly connected to the objective1, said combination being stabilized. The mirrors 24 and 25 at rightangles to each other are fixed to the casing (not shown) and are thusunstabilized. As seen, the fundamental construction is exactly the sameas in FIG. 1, and consequently the pivot point for camera stabilizationhas to be where the focus of the objective would have been located ifthe unstabilized mirrors 24 and 25 are supposed removed, point A. In thecase of dimensioning for telescope stabilization the pivot point must inthis case be divided into two axes 26 and 27 at right angles to eachother and spaced apart, but still the requirement is valid that theseaxes are to intersect the line L. It must be noted that here balancemust exist about both axes, separately.

The stabilized part alone must thus be balanced around an axis and thestabilized part, together with the part of the bearing which isstabilized around only one axis, corresponding to the "middle part" of acardan ring, must be balanced around the other axis. The pivot axis 26,which is parallel to the intersecting line 28 of the unstabilizedmirrors, is to be placed so that the axis 26 goes through point B, andthe pivot axis 27, which is parallel to the intersecting line 29 of thestabilized mirrors, is to be placed at point C. The two pivot axes 26and 27 are thus to be placed at the distance f_(ok) from the focus A ofthe objective 1, where this focus would have been located without theunstabilized mirrors 24 and 25, but on either side of this point.Consequently, the distance between the two pivot axes will here be equalto 2 f_(ok), which may cause some complication at the balancing.Therefore, it will probably in practice be a good alternative in thiscase to dimension for camera stabilization and accept the smalldeterioration which results and which manifests itself such that theimage is seen to be stable in relation to the field stop and not inrelation to the environment. In practice this is of importance only whenthe casing makes extremely rapid movements, such as may be the case, forexample, when the instrument is fixed to a vibrating support.

If for this reason dimensioning for camera stabilization is notsuitable, other methods may be used where the disadvantage of separatepivot axes is avoided. Dimensioning for telescope stabilization need notbe brought about here by moving the pivot point or both pivot axes fromthe point realizing dimensioning for camera stabilization but, forexample, by placing a lens (or several lenses) such that the requirementthat light from an observed object first is to pass only fullystabilized optics and then is to pass only unstabilized optics is notsatisfied (the flat covering disc in front of the objective isneglected). Accordingly, for example, a negative lens stabilized aroundone or two axes can be placed so that the light passes this lens beforeit passes the ocular, or the flat covering disc in front of theobjective may be replaced placed by a positive lens which, if desired,may form part of the objective lens system. It should be noted that ifthe methods exemplified above are used, the pivot point generally cannotbe placed exactly on line L. These small modifications which preferablyinfluence the degree of stabilization to only a relatively slight extentare intended to be included in the invention, but the gain will probablyin most cases be small compared to the increase of complexity. The"extra" lenses may also introduce aberrations.

Still another method of realizing dimensioning for telescopestabilization in structures where the pivot point is placed for camerastabilization should be mentioned. This method is aimed at getting thestabilized part, by servo mechanisms or other means, to move with acertain fraction of the turning movements of the casing so as in thisway to modify the degree of stabilization with a certain factor. It wasmentioned above that this factor is to be equal to (1±1/F), where F isthe magnification of the telescope and where the minus sign applies toinstruments in which the image is seen upright. If this is the case,then the following relation must be true: φ=α/F, where α is the angleturned by the casing and φ is the proportional movement of thestabilized part in the same direction.

Below a construction is described in connection with FIG. 7, whichpermits a very great angular deflection at the same time as the mirrorspresent remain relatively small depending on the extremely smalldistance between the pivot axes and the unstabilized mirror. Thetelescope is here assumed to be held horizontally and the angulardeflection equals zero. The optical axis is then situated wholly in thehorizontal plane. The mirror 35 is unstabilized and the stabilized partaccordingly consists of the objective 1 together with a pair of roofmirrors 37 and a mirror 38. The pair of roof mirrors 37, which may bereplaced by a corresponding prism, consists of two mirrors 39 and 40 atright angles to each other, the intersecting line 41 of which is in thehorizontal plane.

This kind of optics is described in detail in U.S. Pat. No. 3,298,770.For camera stabilization the pivot point, according to the general rulepreviously described, has to be at point A, where the focus of theobjective would have been located if the mirror 35 would not haveexisted. In the case of dimensioning for telescope stabilization thepivot point must here be divided into two axes 42 and 43 at right anglesto each other, where the axis 43 affords freedom of rotation in thehorizontal plane and the axis 42 in the vertical plane. As is seen inFIG. 7 the pivot axis 43 is displaced from point A the length f_(ok)towards mirror 35 along the line L and the pivot axis 42 is likewisedisplaced the length f_(ok) from point A, but in this case parallel withthe visual line along the line L. The relations set forth above are trueirrespective of the angle β (see FIG. 7).

If the use of different pivot axes according to the above is notdesired, there is of course the possibility of placing the pivot pointfor camera stabilization and introduce proportional influence asdescribed before. Another possibility is to place the pivot point whereaxis 43 and line L have their intersection. As the pivot axis 43 then isplaced for telescope stabilization in the horizontal plane, only verylittle proportional influence is needed in the vertical plane only, togive exact telescope stabilization also in this plane.

It would carry up too far here to describe every conceivableconstruction of optics with stabilization according to this invention,since an almost unlimited number of variations is conceivable. If theexamples of dimensioning described above cannot be used directly,analysis of the properties possessed by the optics has to take place inevery particular case when dimensioning for telescope stabilization. Inthe case of camera stabilization the pivot point as a rule has to bewhere the focus of the objective would have been located if theunstabilized optical element would not have existed, provided that lightfrom the observed object first passes only fully stabilized optics andthen only passes unstabilized optics. The unstabilized flat coveringdisc in front of the objective can be disregarded here.

An example of a pair of simple binoculars with image stabilizationaccording to the present invention is described below in connection withFIGS. 8 and 9, where the binoculars are supposed to be held horizontallyand the angular deflection equals zero. The optics for image reversal inthe two parts of the binoculars is here essentially of the porro type.Rays from an observed object passes, in the left part of the binoculars,first the objective 1', is then reflected in the mirrors 52 and 53stabilized together with the objective, before they reach the ocular 9'after reflection in the unstabilized mirrors 55 and 56 at right anglesto each other. A corresponding path of rays exists in the right part ofthe binoculars. Thus, the stabilized part consists of the two objectives1', 1" and the mirrors 46, 47, 52 and 53. This unit has a cardanmounting at the pivot point A which is placed for exact camerastabilization of the two parts of the binoculars; consequently, thefocal points of the two objectives, where they would have been withoutthe mirrors 48, 49, 55 and 56, meet at the same point where the pivotpoint A is situated. As seen in FIG. 9, the optical axes are here whollyin the same plane, the horizontal plane, before reflection occurs in theunstabilized mirrors.

If dimensioning for exact telescope stabilization is necessary,proportional influencing of the stabilized part may be used withadvantage, as this method for natural reasons cannot give rise topossible aberrations or doubling of the images.

In order that the construction described above may be usable inpractice, there is, like in any other binoculars, required a mechanismto make a variable ocular distance possible.

An example of a simple form of such mechanism is described below whichhas the advantage that no further optical elements are introduced andthat only one movable part is used. This part consists of an oculartogether with corresponding unstabilized pairs of mirrors, and thiscombination is rotatable around a vertical axis placed for telescopestabilization. If the right ocular 9" is chosen to be adjustable, thisocular and the mirrors 48 and 49 thus have to be rotatable as a unit asmall angle around the vertical axis 58 (FIG. 8) which intersects line Land the distance of which to the pivot point A has to be as large as thefocal length f_(ok) of the ocular.

Although the binoculars described above are advantageous by theirsimplicity, they have a disadvantage which may be troublesome in certaincases; angular deflection in the vertical plane results in rotation ofthe image in different directions in the two parts of the binoculars,said rotation being dependent on the angular deflection and the angle γ,see FIG. 8. A method of handling this problem is to minimize this angle,which may be achieved if an unstabilized rhombic prism 59 according toFIG. 10 is introduced, so that the necessary distance between theoculars 9' and 9" is maintained.

If the angle γ is minimized in the way described above or otherwise, therotation will be so small that in most cases it will probably beacceptable. It must be noted that when the angular deflection in thevertical plane is small in relation to the maximum value, which mostoften is the case, the rotation is also correspondingly lesser and willbe zero when the angular deflection is equal to zero. At the centre ofthe field of image the two images are always in full accordance.

There is, however, a method of entirely eliminating the above problem,aiming at controlling the image rotations in such a way that they occurin the same direction and are equally great in the two parts of thebinoculars. An example of this is shown in FIG. 11 where the optics inprinciple is the same as that of the telescope described in connectionwith FIG. 7. The two objectives 1' and 1", the mirrors 64, 65, 66 andthe pair of roof mirrors 67 are rigidly fixed to each other andstabilized around the pivot point A which is placed for exact camerastabilization of the two parts of the binoculars. The requirement of anequally great rotation in the same direction taking place in both partsof the binoculars is that the angles ε and β (see the figure) are equaland that an even/uneven number of reflections occur in the unstabilizedmirror systems 69 and 70. As seen, this requirement is here satisfied;the pair of roof mirrors 69 reflects the light two times, the mirror 70is entirely plane, consequently the light is reflected here only once. Avariable ocular distance can be realized here as has been describedabove; which part of the binoculars is chosen for this is hereunessential, since the plane mirror 70 and the pair of roof mirrors 69here give exactly the same result at a rotation about a vertical axis. Apossible dimensioning for telescope stabilization may suitably beachieved also for this construction by proportional influencing of thestabilized part.

Several advantages are common to solutions of the type described above;if the pivot point is placed at the natural centre of gravity of thestabilized part, the need for a counterweight is eliminated, which meansthat a practical instrument can be constructed about as small and lightas the corresponding unstabilized binoculars. As seen the optical andmechanical complexity is drastically reduced compared to existingimage-stabilized binoculars. Contributing to this is the construction ofthe stabilized part which permits that objectives of a standard type maybe chosen without the introduction of image stabilization deterioratingthe image quality.

Even if the invention has been described and shown with reference tocertain embodiments, it is obvious that the invention is not limited tothose disclosed in the specification and in the drawings and that manymodifications thereof are possible for professional men.

I claim:
 1. An image-stabilized optical instrument defining an opticalpath for the passage of optical rays therethrough, comprising a casing,an objective mounted in said casing in said optical path, a firstoptical element mounted in said casing in said optical path, said firstoptical element including at least one plane reflecting surface, asecond optical element mounted in said casing in said optical path, saidsecond optical element including at least one plane reflecting surfaceand being unstabilized and so substantially following the movements ofthe casing, at least a part of the objective being rigidly connectedwith the first optical element to form a stabilized unit which isrotatable in relation to the casing around at least one axis.
 2. Aninstrument according to claim 1, wherein the stabilized unit isrotatable around at least two axes and said first optical element servesas a counterweight to said part of the objective.
 3. An instrumentaccording to claim 2, wherein each of said axes passes substantiallythrough the imaginary straight-line extension of the optical axis of theobjective through the second optical element.
 4. An instrument accordingto claim 3, wherein said axes meet at the same point lying substantiallyat the focus of the objective on said imaginary extension.
 5. Aninstrument according to claim 1 or 2, wherein the first element and thesecond element each comprise a pair of reflecting surfaces placed at anangle to each other.
 6. An instrument according to claim 1, 2, 3 or 4,wherein said stabilized unit is arranged to rotate, within a frequencyrange of the rotational movement of the casing, around at least one axiswith an acceleration substantially proportional to but different fromthe acceleration of the rotational movement of the casing in relation tospace.
 7. An instrument according to claim 5, wherein said stabilizedunit is arranged to rotate, within a frequency range of the rotationalmovement of the casing, around at least one axis with an accelerationsubstantially proportional to but different from the acceleration of therotational movement of the casing in relation to space.
 8. Animage-stabilized binocular type optical instrument defining first andsecond optical paths for the passage of optical rays therethrough,comprising a casing, a pair of objectives respectively mounted in saidcasing in said optical paths, a first pair of optical elementsrespectively mounted in said casing in said optical paths, eachincluding at least one plane reflecting surface, and a second pair ofoptical elements respectively mounted in said casing in said opticalpaths, each including at least one plane reflecting surface, said secondoptical elements being unstabilized and so substantially following themovements of said casing, at least a part of each objective beingrigidly connected with one of said first optical elements to form astabilized unit and said units being rigidly connected to each other andbeing rotatable through a limited angle in relation to said casingaround at least one axis.
 9. An instrument according to claim 8, whereinthe objectives have optical axes imaginary extensions of which meet at apoint.
 10. An instrument according to claim 9, wherein the stabilizedunits are rotatable around two axes, both intersecting the imaginaryextensions of the optical axes of the objectives.
 11. An instrumentaccording to any of claim 8 to 8, wherein the second unsatabilizedoptical element in one of said light paths includes an even number ofreflecting surfaces and the second unstabilized optical element in theother of said light paths includes an odd number of reflecting surfaces.12. An instrument according to claim 11, wherein said stabilized unit isarranged to rotate, within a frequency range of the rotational movementof the casing, around at least one axis with an accelerationsubstantially proportional to but different from the acceleration of therotational movement of the casing in relation to space.
 13. Aninstrument according to claim 8, 9 or 10, wherein said stabilized unitis arranged to rotate, within a frequency range of the rotationalmovement of the casing, around at least one axis with an accelerationsubstantially proportional to but different from the acceleration of therotational movement of the casing in relation to space.